The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
Plastic films with thermal adhesive backings are used for decorating a wide variety of materials and products. While these films are sometimes referred to as heat transfer vinyl, modern heat transfer films are usually manufactured from urethane, polyester, or polypropylene to avoid the toxicity of vinyl chloride when exposed to heat.
Cut thermal adhesive films are composed of at least two layers, the plastic substrate, and a thermal adhesive coating. The substrate often contains a colorant or additional decorative additives. The films are usually affixed to a backing paper or plastic substrate. Films of this type may be cut with a plotter/cutter machine so that regions of the cut film may describe lettering or a decorative shape or image. Often the films are cut in a manner that leaves the backing film or paper intact. The type of cutting is called a ‘kiss cut’.
After cutting, the portions of the film that do not include the lettering or shape to be transferred must be removed. The non-transferred portion of the film is called the ‘negative area’. Removing the negative areas of the film for all but the simplest designs is performed by heat press operators as a manual process. The manual removal of negative areas of heat transfer films is called ‘weeding’. After weeding, the positive areas of the film are transferred using a heat press to the surface of the material or product to be decorated.
The labor costs for weeding constrains the complexity of designs that may be used for decorating objects with heat transfer films. While there are other processes such as pad printing, screen printing, and direct inkjet printing that may easily handle complex designs, heat transfer films may be formulated to adhere to and decorate a much wider variety of materials and combination of materials found in products than other decorative printing processes. Heat transfer films may also be manufactured with metal films or flakes, or be embossed to form other optically active surfaces.
While automated processes exist for selective weeding, each process suffers from significant drawbacks as described below.
One method for removing negative design areas prior to heat transfer is laser weeding. Laser weeding involves using laser ablation to remove negative design areas. Laser ablation is a process by which a portion of a material is removed by rapidly heating the material with light from a laser so that a portion of the material is converted directly from a solid to gas. Specialty films have to be manufactured to use the laser cutting and ablation process. The heat transfer film is manufactured so that its transfer characteristics are preserved, and is designed to minimize the toxicity of the gas resulting from ablation.
The laser weeding method contains significant drawbacks. Films with large variation in surface may be difficult to process. The reflective properties of the film must be designed so that laser ablation may take place. Because the films require specialized designs, many metallic or holographic films are difficult or impossible to weed in this manner. Additionally, laser ablation requires that the full negative area be traversed, rather than just the cut edge. This can greatly increase the cutting time for processing thermal transfer films. Thus, laser weeding is constrained to small designs using a constrained set of colors and surfaces.
Another method for removing negative design areas is selective thermal transfer. Selective thermal transfer involves designing a pigmented film that separates easily when a portion of it is subjected to selective heat application. A printing process may transfer pigmented waxes or thermoplastics directly from a carrier sheet or ribbon to a substrate. Specialty transfer materials may be manufactured with transferable dots, segments, or regions.
The selective thermal transfer technique also contains significant drawbacks. Because of the requirement for specially designed materials, certain types of optical effects are difficult to achieve. Additionally, constraints are placed on the durability of the material used for the transfer. Selective thermal transfer may also be imprecise and may result in broken or approximate edges for the transferred design. Finally, while heat presses can be created that conform to a variety of shapes and curvatures, building a machine for selective application of heat transfer for custom shapes or surfaces may be prohibitively expensive.
Masked thermal transfers may also be used to block the transfer of negative design areas. Using a masked thermal transfer is similar to the selective thermal transfer in that it involves using a film that separates easily when a portion of it is subjected to selective heat application. Masked thermal transfer also involves producing or printing a mask which can block the application of the negative portions of a design from the substrate. As with selective thermal transfer, masked thermal transfer techniques are constrained in the types of material that may be used to create the designs, thus limiting the durability and optical effects of the design.
Given the versatility of heat transfer films, it is useful to devise a system and method for the automated removal, or weeding, of the negative design areas prior to heat transfer that is integrated into the application of the cut thermal transfer film and that can be applied accurately to a large range of materials.